A brief tour of the lung

When a person inhales, air flows through the windpipe, through the bronchi, and into smaller tubes known as the bronchioles. Bronchioles lead to small sacs called alveoli where gas exchange occurs. The figure below shows to the major parts of the lung:

A closer look at the alveoli reveals how the gas exchange process functions. The walls of the alveoli, known as alveolar epithelium, are comprised of type 1 cells with long cytoplasmic extensions. Alveolar epithelium are covered with capillaries with a thin blood gas barrier that permits gas exchange to occur. Before reaching this barrier, gas diffuses through a liquid layer secreted by type 2 cells. Contained in the liquid layer, a phospholipid called lung surfactant stabilizes each alveoli during the respiration cycle. Such a layer develops during a baby’s first few hours of life creating a thin foam essential for proper lung function. A more detailed discussion of lung surfactant’s role in stabilizing the lung from collapse follows in this article. The figure below shows a cross section of an alveoli. Lung surfactant is a thin layer that lines the interior of the alveoli.

Early assessment of lung form

Early examinations of lung foam focused on lung extracted from animal dissections. Scientists found these foams to be unusually stable and resistant to silicon antifoams. Simple agitation of blood serum or oedema fluid (a fluid pools in the lungs during some illnesses) creates less stable foams, indicating that lung tissue is not simply comprised of these materials. In contrast to simple forms, bubbles in lung foam retain their size when placed in water for longer periods of time suggesting that they have zero surface tension. Bubbles in oedema or serum foams quickly disinigrate in water due to excess internal pressure [1].

Low surface tension between lung capillaries and alveoli is necessary for normal lung function. If the surface tension between these two regions were high, fluid from the capillaries would be sucked into the alveoli preventing normal respiration. An insoluble protein lining on the outside of the alveoli would reduce this surface tension. Estimates of the surfactant monolayer's thickness were made by drying and weighing lung tissue samples and assuming known bubble sizes. The monolayer was found to be about 50 angstroms thick [1].

Role of surfactant in stabilizing lung foam during respiration

Lung surfactant forms a thin monolayer at the alveoli liquid-air interface, and lowers the surface tension from that of normal water (~70 dyne/cm) to close to zero as a human exhales [2]. Functioning lung surfactant must have a minimum surface tension of 10 dynes/cm when compressed to prevent alveolar collapse [2]. Surface tension in this monolayer increases as the area available area for the monolayer decreases. From experiments, the surface tension T of lung surfactant was found to have the following relationship with area of an alveoli [6]:

<math> T = 10 + 36(A/A_0)</math>

where <math>A_0</math> is the maximal volume.

Laplace’s law for a spherical structure of radius r gives the following relationship between internal pressure P and surface tension T:

<math>P = 2T/r.</math>

Combining these two equations yields the pressure as a function volume for alveolus. The figure on the left below shows this theoretical curve based on an experimentally measured constant surface tension of 46 dynes per centimeter during expansion [6]. During exhalation, the stability of alveoli relies on its ability to maintain its volume as the pressure decreases. The condition for constant volume dP/dV > 0 is equivalent to the product of the compressibility K and surface tension T being less than a dimensionless constant k which depends solely on the geometry of the surface:

For a spherical geometry k=2. The figure on right show the dividing curve for alveoli stability. Combinations of surface tension and compressibility that lie above this curve will not be able to maintain a stable volume. It is only when the surface tension of lung surfactant drops to 11 dynes/cm that the alveoli become unstable [6].

Lung surfactant must change the surface tension appropriately with changes in air pressure to maintain a consistent radius, preventing a collapse of the alveoli during the respiration cycle.

This figure demonstrates alveoli function with and without proper lung surfactant.

In addition to preventing alveoli collapse, reductions in the surface tension reduce the amount of energy necessary to inflate the lungs, making breathing easier [3]. Lung surfactant deficiencies are associated with a range of lung problems including oxygen starvation, lung collapse and pulmonary edema [3]. Patients with acute respiratory distress syndrome (ARDS) often have lung surfactant that lacks the full range of surface tension modulation upon compression [2]. This disease has a 30-50% mortality rate and is found in children and adults. Although this disease is caused by improperly functioning lung surfactant, patients respond poorly to replacement lung surfactant (RLS) therapies that are successful in premature babies born without lung surfactant [2]. Doctors believe that ARDS in adults may result from surfactant inactivation caused by plasma and inflammatory leaking into alveoli [4]. Such processes dramatically alter the surfactants' response to changes in pressure and reduce lung function. Some research on synthetic lung surfactants also focuses on understanding surfactant inactivation. Treatments that specifically target reducing surfactant inactivation would be more effective in adults than replacement lung surfactant therapies that were developed to provide premature babies with lung surfactant.

Components of lung surfactant

Proper lung surfactant is a complex mixture of roughly 90% lipids and 10% proteins in an aqueous solution. For most animals, DPPC (dipalmitoylphosphatidylcholine) comprises between 40% to 80% of the lipids in weight. The rest are unsaturated phosphoatidylcholines, saturated and unsaturated phophatidynlglycerols, other anionic lipids and cholesterol [5]. Four identified proteins are creatively named SP-A, SP-B, SP-C and SP-D. Only SP-B and SP-C are amiphilic. Replacement lung surfactants often lack water soluble SP-A and SP-D proteins because they do not affect surface activity and are thought to be involved in immune system [7]. Doctors believe that a person's immune system might react to the presence of proteins involved in the immune response of other animals. Although it is not believed to be part of the surface layer, SP-A may also play a role in surfactant transportation and adsorption [7]. In this role, it could regulate the surface layer's composition.

Synthetic lung surfactants

Replacement lung surfactant (RLS) therapies are often refined from either bovine or porcine lungs due to limited human lung surfactant supplies. Since most RLS are generated from biological sources, virus transmission from animals to humans is a risk, and manufacturing costs are high. On going research focuses on developing cheaper, safer synthetic lung surfactants and adding synthetic additives to biological based lung surfactants. Proteins are often removed from animal derived lung surfactants during processing for use in humans for safety concerns. Often this affects a surfactant's properties, and ongoing research also examines how additives may improve an RLS's effectiveness.

Properties of a functioning lung surfactant

Any functioning replacement lung surfactant must have three main properties that ensure proper lung function. It must have a low surface tension on compression to prevent alveoli from collapsing. At the same time, RLS must remain viscous to avoid leaking out of alveoli after exhalation. During inhalation, lung surfactant must quickly adsorb to and re-spread along the interface [7]. Native lung surfactant beautifully balances these key features.

Measuring properties of thin layers

Traditionally the phase behavior of a lung surfactant's thin monolayer was studied using a Langmuir trough method. More recently, experiments have measured expansion and contraction of captive bubbles to probe the same behavior.

-Langmuir Trough

A Langmuir trough is often used to measure the surface tension of a thin amphiphillic films. Typically this device consists of a teflon trough partially filled with water with a movable barrier at one end. A thin film is generated by dissolving a known amount of apmhiphillic molecules into a spreading solvent. This mixture is dropped into the trough, and the solvent is permitted to evaporate, leaving a thin film of amphiphillic molecules behind. The figure below shows an example of a Langmuir trough.

Adjustments to the area may be made by moving the barrier on one end. Surface tension is measured by a lateral pressure sensor at the other end of the trough. An example of data taken with a Langmuir trough is shown below [7]. Surface tension was measured as a function of monolayer area. A sample of a commercial replacement lung surfactant, Infasurf, was cycled three times. While the areas corresponding to plateau values vary for each cycle, the first plateau occurs at surface tension ~40-45 mN/m corresponding to removal of fluid phases, and the second plateau at 65 mN/m indicates the monolayer collapse.

-Captive Bubble

While Langmuir troughs provide a good method to measure the surface tension in flat two-dimensional thin films, a captive bubble technique measures the surface tension of surfactant bubbles in round shapes. This replicates conditions in pulmonary systems more effectively than Langmuir trough techniques. Often surface tensions measured using this method are slightly lower than surface tensions measured using Langmuir troughs.

In the captive bubble method, a small air bubble is generated in a closed system of liquid that contains a small amount of surfactant. The pressure of this closed system may be varied. As surfactant moves to the liquid-air interface of a bubble, a fraction of created bubbles will stabilize. The radii of stable bubbles depend on the pressure and surface tension of surfactant along the interface, and surface tension may be easily calculated using Laplace's equation discussed earlier. Surface tensions curves as a function of pressure may be easily measured using this method. Since such devices are relatively easy to clean, this method may be used to characterize how different synthetic additives may affect surfactant performance.

The figure below shows how a bubble's shape varies under a range of conditions: (a) initial bubble at atmospheric pressure in absence of surfactant, (b) pressure lowered without a significant amount of surfactant present at interface, (c) bubble with surfactant along liquid-air interface, and (d) bubble with surfactant at pressure above atmospheric. As one can see, the shape of the bubble varies with amount of surfactant along interface and applied pressure to the system [8].

Potential for monolayer composition variation over respiration cycle

Wide variation exists among replacement lung surfactants on the market, and even different batches of the same replacement lung surfactant can differ widely [2]. Although the composition of the overall surfactant in the human lung is known from analyzing minced lung samples, the precise composition in the monolayer liquid-air interface may differ from the lungs overall composition [7]. Some researchers have even suggested that monolayer composition may vary over the course of a respiration cycle [7]. Exchange between dissolved surfactant and monolayer surfactant provides one mechanism for local variation in monolayer composition. If correct, this theory makes production of replacement lung surfactants challenging because the ideal composition remains unknown. A new push in this field is to understand the role that each individual lipid and protein play in functioning lung surfactant in hopes that it will shed more light on this question [7]. At this point in time, the techniques to examine lung surfactant composition over a respiration cycle does not exist and remains elusive.

From a start in trying to replicate lung foams by agitating oedemic fluid and blood, this field had made progress in understanding basic lung form and surfactant function. Early breakthroughs culminated in the creation of effective replacement lung surfactants that save the lives of premature children. Current research focuses on refining replacement lung surfactant therapies to be more effective in children and develop ones that work for acute respiratory distress in adults as well. A recent suggestion that lung surfactant composition might change over the course of the respiration cycle has interesting implications for further understanding lung form and developing treatments for lung diseases. Only time will tell if this idea will have any merit.